In fields where strength and weight reduction of materials are required, for example in the field of fuselage of an aircraft, in order to meet such demands, the use of many composite materials such as CFRP (Carbon Fiber Reinforced Plastics) is essential. In order to maintain a high level of reliability of structures made from such composite materials and to perform more efficient design work, damage detection technology (health monitoring technology) is attracting much attention. As devices for performing this kind of detection of damage and defects in composite materials, there are damage detection devices as disclosed in patent publication 1 and 2 that use a FBG (Fiber Bragg Grating) optical fiber sensor. Recently, optical fibers are becoming very thin (for example, 52 μm diameter). As a result, even when embedded in a structure, there is not much of a decrease in strength of the structure. Therefore, optical fibers have an advantage that they have a high degree of freedom regarding placement.
The inventions disclosed in Japanese patent publication 1 and 2 below use an optical fiber sensor having a grating section wherein piezo elements that are fixed and arranged at specified locations in a structural composite material, lead wires that transmit signals to the piezo elements, an optical fiber sensor attached to the structural composite material so that the composite material of the structural composite material is located between the optical fiber sensor and the piezo elements, the optical fiber sensor having a grating section to reflect light of a specified wavelength to a core section, a light source that shines light on the core section, and a characteristic detection unit that detects a characteristic of the reflected light from the grating section, and vibrating the material by the piezo elements, detects damage from the change in output from the characteristics detection unit. A spectrum analyzer that detects the frequency characteristic of reflected light from the grating section is used as the characteristic detection unit.
Furthermore, in the invention disclosed in Japanese Patent Publication No. 2005-098921, a comparison is performed with detected data of a normal structural composite material that was acquired beforehand. Alternatively, another method is disclosed in which in the frequency distribution that is detected by the spectrum analyzer, a threshold value is set for the fluctuation value from when there is no oscillation of a specified frequency, and when the detected value is equal to or less than that threshold value, it may be determined that there is damage (paragraph 0032).
In the invention disclosed in Japanese Patent Publication No. 2007-232371, two optical filters are provided in a spectrum analyzer. It is proposed that by outputting reflected light to an arithmetic processing unit via the two optical filters, the spectrum analyzer will detect a wavelength oscillation signal of the reflected light with high sensitivity. It is also proposed that the arithmetic processing unit will calculate a value (DI value) that corresponds to the scale of the damage of the test object based on the obtained wavelength oscillation signal.
As a method of damage detection technology, research is being performed regarding a method in which ultrasonic waves called Lamb waves are generated and detected, and the occurrence of damages is diagnosed based on the change in the detected waves. The Lamb wave is an ultrasonic wave that propagates through a thin plate, and propagates over a long distance with a relatively small amount of damping. Therefore, it is a form of ultrasonic wave propagation that is suitable to damage detection. Moreover, Lamb waves have two characteristics; a multi-mode characteristic and velocity dispersion characteristic (frequency dependence), and depending on the plate thickness and frequency, there are plurality of modes having different speeds. Due to these complex characteristics, conventionally, damage detection was performed by using only information about a specific frequency of the Lamb waves.